This invention relates to boilers and, in particular, to condensing boilers typically used for heating water for commercial or domestic applications such as space heating and domestic water heating.
Most heating systems using gas or oil combustion to operate at relatively low fuel efficiencies in order to exhaust flue gases well above the condensation point. This is done through sizing and configuration of the heat exchanger, to ensure that the temperature, time and turbulence factors affecting thermal transfer are sufficiently low as to provide exit flue gas temperatures above 270xc2x0 F. Some units apply active anti-condensation measures such as supply water diversion/recirculation valves to maintain minimum return water temperatures above the condensation zone (e.g. 100xc2x0 F. -140xc2x0 F.). This is done to avoid the need to handle condensates with their inherent corrosive qualities, allowing the use of simple cast iron heat exchanger construction. Essentially this means losing 16-20 percent of the energy of the fuel which simply goes up the stack.
It has been recognized that condensing systems are inherently more efficient because they convert the latent heat of condensation into useful energy instead of expelling it with the exhaust. However liquid byproducts of the combustion must be contained and channelled away for disposal. Heat exchanger materials must be capable of withstanding the corrosive products encountered during wet-dry cycling. Steps should also be taken to ensure that the burner and igniter systems, along with other system elements such as sensors, are not fouled by moisture or condensation.
In a condensing furnace or boiler, water droplets are deposited on heat exchanger walls in zones where the flue temperature has cooled below 140xc2x0 F. During normal firing/heating operation, the wet zone would typically be spatially removed from sensitive hot zone components such as the burner and ignitor. However significant moisture often remains on a condensing heat exchanger""s cool zone surface after burner shutdown. Such moisture can migrate within the heat exchanger and venting system following re-evaporation, driven by residual heat in the heat exchanger. Related convective air movements within the heat exchanger plenum can run counter to the normal on-cycle flow direction in a typical heat exchanger configuration which incorporates downflowing flue gases and condensates.
The typical response has been to use a two stage heat exchanger to separate the wet (condensate) zone from the igniter/burner area. The use of primary and secondary heat exchangers has been adopted to isolate the condensation from the combustion zone and limit the extent of corrosion resistant materials required. This approach is sensible for hot air furnaces where the components are relatively inexpensive and the added cost of a second stage of heat exchanger is not excessive.
However the use of primary and secondary heat exchangers is more burdensome in a hot water heating system with a boiler, due to American Society of Mechanical Engineers (ASME) pressure containment requirements.
Some recently introduced condensing boilers use dual heat exchangers. Such products employ a relatively conventional primary heat exchanger with an integral or mated/external xe2x80x9cpupxe2x80x9d unit built to handle corrosive moisture. A trap is typically placed between the primary and secondary heat exchangers to block back flow of liquid flue products. Such an air trap may take the form of a mechanical vent damper or a thermal air trap. These occupy a significant space, rendering these products bulky and expensive to ship and install. The dual heat exchanger approach is typically costly, requiring two ASME pressure vessels plus communicating flue gas passageways.
A previous approach involved the use of a single section heat exchanger of copper or cupra-nickel. Such units employed direct spark ignition, a single speed AC blower and a simple control/safety circuit. A pre-purge is used to clear exhaust venting according to code requirements, but typically all activity terminates with the end of a call-for-heat, when an open thermostat cuts off the fan. Such units have suffered from metallurgical deterioration when copper breaks down under acidic attack to form sulfates or sulfides, leading to plugging of fluid paths and spark plug fouling due to the spark plugs becoming wet and corroded. In an attempt to overcome these problems, some units switched to hot-surface ignition (HSI). However in such cases the HSI element itself often failed, apparently due to moisture infiltration into its ceramic body.
Fans and controls for such heating appliances are typically unsophisticated, being unable to deliver air at varying flow rates or maintain control over fans and pumps in the post-firing period. Fans are typically of a split capacitor or shaded pole AC variety with little potential for variable speed. Recent developments with electronic frequency modulation provides some throttle range for AC motors. However turn down ratios of 3:1 are the typical limit.
If employed at all, a post purge cycle has been used to evacuate exhaust from the vent to reduce off-cycle condensation with cooling in the stack (which often cannot handle corrosives). However, controls available with existing boilers rarely offer post-firing functionality.
There is provided, according to an embodiment of the invention, a condensing boiler which includes a heat exchanger and having a top and a bottom. There is a burner adjacent to the top of the heat exchanger. An exhaust for spent flue gas and condensate is adjacent to the bottom of the heat exchanger. An intake system includes an air conduit, a fuel inlet and a fan which supplies a fuel/air mixture to the burner during a heating mode and which supplies air to the heat exchanger during an idle mode. Controls modulate the fan so that air flow to the heat exchanger during the idle mode is sufficient to inhibit upward migration of condensate through the heat exchanger, but insufficient to cause significant outflow of air through the exhaust.
Preferably the fan is a variable speed fan. For example the fan may have a DC brushless motor. In one example the fan has a maximum rotational speed during the idle mode which is generally 0.02 of its maximum rotational speed during the heating mode.
The controls may be microprocessor based with a capacity for transmission of pulse width modulated (PWM) signals
In one example the heat exchanger includes coils of metal tubes. The coils may be of stainless steel.
According to another aspect of the invention, there is provided a method of inhibiting corrosion of burners and igniters in condensing boilers having heat exchangers with a burner and an igniter near the top of the heat exchanger, a condensate exhaust adjacent to the bottom of the heat exchanger, and a fan for supplying an air/fuel mixture to the burner during a heating mode when the fan operates at a relatively high rotational speed. The method comprises operating the fan at a significantly lower rotational speed during an idle mode following the heating mode where the burner is off, the rotational speed of the fan during the idle mode being sufficient to inhibit upward migration of reevaporated condensate through the heat exchanger, but insufficient to cause a significant outflow of air through the condensate exhaust.
The idle mode may continue for a specified period after the heating mode if the boiler does not return to the heating mode. For example, the specified period may be 90 minutes.